1. Field of the Invention
This invention relates to liquid crystal devices. In particular the invention relates to liquid crystal devices incorporating ferroelectric liquid crystals, such devices having the advantage over devices using non-ferroelectric liquid crystals in that the interaction between a ferroelectric liquid crystal and an external electric field causing the device to switch is linear.
2. Description of the Related Art
In a paper by N. A. Clark and S. T. Lagerwall published in Applied Physics Letters, Volume 36, pages 899-901 in June 1980 there is described an example of a known liquid crystal device incorporating a ferroelectric liquid crystal.
FIG. 1 is a schematic diagram of the prior art device described in this paper; and
FIGS. 2 to 5 illustrate various polarisation states of prior art device shown in FIG. 1.
Referring to FIG. 1 the device comprises two parallel glass plates 1, 3 enclosing a chiral smectic C phase (C*) liquid crystal 5, the spacing between the plates being less than the distance at which the liquid crystal helix forms in the absence of an electric field. The planes of constant number density of molecules i.e. the smectic layers 7 are chosen to be perpendicular to the plates 1, 3, the internal surfaces of the plates being treated in such a way that the average direction of the molecules of the liquid crystal, i.e. the directors, lie in planes parallel to those of the plates 1, 3. For sufficiently small interplate spacings, these conditions cause the liquid crystal helix to unwind, the liquid crystal having two alternative equal energy configurations corresponding to two different director orientations within each of the planes parallel to the plates through the crystal. These are shown as arrows 9, 11 in FIG. 1, and correspond to the intersection of the cone of angle .theta..sub.s, where .theta..sub.s is the liquid crystal tilt angle or "cone angle", i.e. the angle between the director and the normal to the smectic layers, with the plane in which the directors lie.
Referring now also to FIGS. 2 and 3, the two different director orientations 9, 11 give rise to respective antiparallel ferroelectric domains of spontaneous polarisation, the directions of the polarisation P.sub.s being tangential to the base of the cone. The polarisation of these domains will thus be normal to the plates 1, 3 the polarisation states shown in FIG. 2 arising from directors parallel to 9 being designated "UP" polarisations, and those shown in FIG. 3 arising from directors parallel to 11 and being designated "DOWN" polarisations. The device may be switched between these polarisation states by application of an electric field between the plates 1, 3.
The device as hereinbefore described suffers from the problem, however, that it is difficult to obtain the director alignment in which the directors are held in planes parallel to the confining plates, whilst still allowing rotation of the directors in these planes between their two allowed orientations. Furthermore, as discussed by S.T. Lagerwall and J. Wahl in the Proceedings of the 1985 International Display Research Conference, pages 213-221, unless the spacing between the plates 1, 3 is kept very small, typically less than 2 .mu.m, further so-called "SPLAY" polarisation states of the forms indicated in FIGS. 4 and 5 are created, these states being energetically favoured for thick liquid crystals. Furthermore, in both the "UP" and "DOWN" states P.sub.s points into one plate 1 or 3 and away from the other plate 3 or 1, whereas in the two "SPLAY" states P.sub.s adjacent the plates 1, 3 may point into both plates or away from both plates. Thus if the plates 1, 3 and any coatings thereon impart some preference concerning whether P.sub.s should point towards or away from the plates 1, 3 then the "SPLAY" states may be favoured. As can be seen from FIGS. 4 and 5, in such "SPLAY" states the average through the cell of the component of polarisation normal to the plates 1, 3 is zero, and there will therefore be no net linear interaction between the liquid crystal and an electric field applied between the plates 1 and 3.